1. Field of the Invention
The present invention relates to production of polarized light exposures and, more particularly, to production of high-intensity and high-frequency polarized light exposures having a uniform intensity and uniform direction of polarization, which are useful in the industrial-scale manufacture of photo-aligned optical films.
2. Related Art
One step in the process of manufacturing optical film typically involves the alignment of the molecules in an optical coating on the film. Such alignment may, for example, be performed using a photo-alignment process that involves passing the coated film (referred to herein as a xe2x80x9cwebxe2x80x9d) through a device that exposes the coated surface to polarized light. This polarized light exposure aligns the molecules in the optical coating in a direction parallel to the plane of polarization, thereby producing desirable optical qualities in the coated film. Optimal alignment is produced when the polarized light is uniform in both intensity and direction of polarization across the web. Furthermore, because the time required to produce the desired alignment is related to the intensity of the light to which the web is exposed, the web may be prepared more quickly if it is exposed to higher-intensity polarized light.
It is therefore desirable, in the preparation of photo-aligned optical films, to expose such films to a uniformly-intense field of polarized light having a uniform direction of polarization. Furthermore, in certain applications it is desirable for the polarized light to be in the ultraviolet (UV) spectrum.
Previous systems for producing photo-aligned optical films have encountered a variety of problems. For example, although conventional sheet polarizers may be used to produce the polarized light exposure, conventional sheet polarizers are primarily effective in the visible light spectrum. Such polarizers typically do not efficiently polarize light in the UV spectrum; they typically absorb about 70-80% of the UV light to which they are exposed. As a result, such polarizers are not well-suited for polarizing UV light. This problem is further compounded in the case of high-intensity UV light. Because of the large percentage of such light which would be absorbed by a conventional sheet polarizer, such polarizers would overheat and potentially melt and/or combust if exposed to high-intensity UV light over any appreciable amount of time.
Some previous systems have employed a xe2x80x9cpile-of-platesxe2x80x9d polarizer to produce polarized light for use in photo-alignment systems. Pile-of-plates polarizers are well-known to those of ordinary skill in the art and are described, for example, in Polarized Light: Production and Use, William A. Shurcliff, Harvard University Press (1962), pp. 78-85. As its name suggests, a pile-of-plates polarizer employs multiple dielectric plates arranged in a pile. Although the plates in such a polarizer may be arranged in a variety of ways that are well-known to those of ordinary skill in the art, pile-of-plates polarizers generally share the characteristic that an unpolarized beam of light that is incident upon the first plate in the pile is partially polarized by each of the successive plates in the pile, producing a transmitted beam of light that is adequately polarized.
The same principle has been used in some photo-alignment systems to polarize a plurality of rays in a beam of collimated light. Such systems typically generate collimated light (by using, for example, a point source of light and a lens) which is then polarized by a pile-of-plates polarizer. The usefulness of such systems is restricted by the fact that a point source of light must typically be used to produce collimated light. This requirement typically limits the size of the polarized light exposure that may be produced by such systems, thereby limiting the size of the web that may be prepared using such systems. The point source requirement further limits the uniformity of intensity and uniformity of direction of polarization of the resulting polarized light exposure, thereby resulting in limited uniformity of alignment of the molecules in the web""s coating.
What is needed, therefore, is a system for producing a large, uniform (in both intensity and direction of polarization), and high-intensity exposure of polarized light. In particular, what is needed is a system that is capable of producing such an exposure of polarized light in the UV spectrum.
In one aspect of the present invention, a system is provided in which an expanded (i.e., non-point) source of light may be used to produce polarized light having a highly uniform direction of polarization. Such polarized light is of particular use in the industrial-scale manufacture of photo-aligned optical films. For example, a web (a film having an optical coating) may be exposed to the polarized light, thereby aligning the molecules in the coating in a direction parallel to the plane of polarization. Alignment of the molecules in the coating in this manner produces desirable optical properties in the coating, as is well-known to those of ordinary skill in the art.
The polarized light produced by the system may be in the Ultraviolet (UV) spectrum without degrading the performance of the system. Furthermore, the polarized light may have a high intensity without degrading the performance of the system. The exposure may also be uniform in intensity and uniform in direction of polarization across the web. The system may be scaled to produce large exposures without sacrificing uniformity of intensity or uniformity of direction of polarization. As a result, larger webs may be photo-aligned using the system.
The system includes a light source, which may be an extended light source. In other words, the light source need not produce collimated light. The light source may, for example, be a source of high-intensity UV light. The system also includes a pile-of-plates polarizer (made, for example, of transparent uncoated glass). The light source produces light that is oriented at or near Brewster""s angle to the pile-of-plates polarizer. The pile-of-plates polarizer polarizes the light from the light source to produce uniformly-polarized light.
The system also includes a film coated with an optical coating. The coated film is referred to herein as a xe2x80x9cweb.xe2x80x9d The coating may be made from any material suitable for alignment by exposure to polarized light. For example, such a coating may be made from Linear PhotoPolymerization (LPP) material. The web may, for example, be fed through an electro-mechanical system that passes the web through the field of polarized light produced by the pile-of-plates polarizer. The web is oriented orthogonally to the pile-of-plates polarizer. As a result, the polarized light exposure produced by the pile-of-plates polarizer on the web has a uniform direction of polarization over large areas across the web, even when the light produced by the light source described above is not collimated.
The uniform polarized light exposure on the web causes the molecules in the web""s coating to align parallel to the exposure""s plane of polarization, thereby producing desirable optical properties in the coating.
Other features and advantages of various aspects and embodiments of the present invention will become apparent from the following description and from the claims.